1. Technical Field of the Invention
The present invention relates to an oil-cooled compressor which is constructed so that oil is fed to a body of the compressor for lubrication, cooling, or shaft sealing. Particularly, the invention is concerned with an oil-cooled compressor in which the discharge temperature of discharge gas is controlled appropriately by controlling the amount of oil to be fed.
2. Description of the Related Art
There is known an oil-cooled compressor constructed such that oil is fed to a body of the compressor for lubrication, cooling, or shaft sealing. An example in which this known oil-cooled compressor is an oil-cooled screw compressor will now be described with reference to drawings attached hereto. FIG. 4 is a schematic system diagram of an oil-cooled screw compressor, FIG. 5 is a graph explaining a relation between a discharge pressure Pd and a power w of a compressor body and a relation between the discharge pressure Pd and an oil quantity q, and FIG. 6 is a graph explaining a relation between the discharge pressure Pd and a discharge temperature Td.
A description will first be given of a conventional oil-cooled screw compressor. The numeral 2 in FIG. 4 denotes an oil-cooled screw compressor. The screw compressor 2 is provided with a compressor body 12 in which a pair of intermeshing male and female screw rotors 11 is accommodated rotatably. A discharge path 13 extends from a discharge port of the compressor body 12, and an oil separation/recovery unit 14 as an oil separating means is disposed in the discharge path 13. An oil separating unit 15 is provided at an upper position within the oil separation/recovery unit 14. A lower portion of the oil separation/recovery unit 14 serves as an oil sump 16 for staying therein of oil after separation by the oil separating element 15. On one end of an oil feed path 18 with an oil cooler 17 disposed therein is connected to the oil sump 16, while the opposite end thereof is in communication with the compressor body 12.
Thus, the oil-cooled screw compressor 2 is constructed so that oil which has flowed through the oil feed path 18 from the oil sump 16 in the oil separation/recovery unit 14 and cooled by the oil cooler 17 is fed to a rotor chamber, bearings and a shaft sealing portion located within the compressor body 12. (The rotor chamber, bearings and a shaft sealing portion are not shown in the figures) An oil quantity q of oil fed to the compressor body 12 of the oil-cooled screw compressor 2 varies depending on a discharge pressure Pd of the compressor body 12. A relation between the oil quantity q and the discharge pressure Pd is as shown by the following equation (1). A nozzle area of a communicating portion of the oil feed path 18 for communication with the compressor body 12 is assumed to be S.q=C1×S×(Pd)1/2  (1)In the above expression (1), C1 is a constant.
The power w of the compressor body 12 can be calculated by the following equation (2):W=C2×{(Vi−κ)/(κ−1)×Ps+Pd/vi}  (2)
In the equation (2), C2 is a constant, vi is an internal volume ratio, κ is a specific heat ratio of air, PS is a suction pressure. The oil quantity q and power w of the compressor body 12 vary as shown schematically in FIG. 5. The discharge temperature Td can be calculated from the following equation (3):Td=w/(C3×q)+To  (3)In the equation (3), To is a feed oil temperature and C3 is a constant.
From the equations (1) and (2) it is seen that the oil quantity q is in a linear relation to the square root of the discharge pressure Pd, while the power w is in a linear relation to the discharge pressure Pd itself. From this fact it can be said that with respect to increase and decrease of the same discharge pressure Pd, the ratio of the increase and decrease quantity q of oil fed to the compressor body is larger than that of the power w. Further, from the equation (3) it can be said qualitatively that the discharge temperature Td rises as the discharge pressure Pd decreases, as shown in FIG. 6.
As to the discharge pressure Pd in the compressor body of the oil-cooled compressor, a maximum discharge pressure Pdmax is established in relation to the specification of the oil-cooled compressor. A higher pressure than Pdmax cannot (or does not) exist. There also is established a lowest discharge pressure Pdmin. A lower pressure than Pdmin cannot (or does not) exist.
As to the discharge temperature Td of discharge gas discharged from a discharge port formed in the compressor body of the oil-cooled compressor, there are established a desirable upper-limit discharge temperature Tdmax and a desirable lower-limit discharge temperature Tdmin. Generally, the upper-limit discharge temperature Tdmax is established (e.g., 100° C.) for preventing the deterioration of oil, and the lower-limit discharge temperature Tdmin is established for preventing the deposition of drain on the discharge side of the compressor body (e.g., 80° C.).
In order to ensure the lower-limit discharge temperature Tdmin at the upper-limit discharge temperature Tdmax, a corresponding value of oil quantity q is determined so as to bring about this state and the discharge pressure Pd is decreased in the state of that oil quantity q. As a result, the discharge temperature Td drops for the reason stated above in connection with the equations (1), (2) and (3). At the initial stage, a certain degree of temperature rise does not give rise to any problem because the discharge temperature is set to the lower-limit discharge temperature Tdmin. As to a more increase of temperature, there can be a case where the temperature rises up to near the upper-limit discharge temperature Tdmax or may exceed the upper-limit discharge temperature, which would cause inconvenience in the operation of the compressor body.
It is preferable for preventing the deterioration of oil that the temperature of oil fed to the compressor body of the oil-cooled compressor be lower than the upper-limit discharge temperature Tdmax, more preferably be maintained at a low temperature. Also, for preventing the deposition of drain from the compressed gas, it is preferable that the oil temperature be kept higher than and close to the lower-limit discharge temperature Tdmin.
Japanese laid-open patent gazette JP-8-4679-A discloses control of the discharge temperature of a compressor in order to prevent the production of drain. However, the compressor in the prior document has a complicated structure which additionally includes a discharge temperature sensor and an oil control valve changing supply oil quantity continuously. In addition, though it is assumed that a complicated control algorithm should be applied for thus complicated structure, the prior document discloses nothing about the control algorithm.